CHRONOMETERS. 83 



ther; and hence a source of irregularity in clocks. 

 Various expedients have been tried for remedying 

 this defect; the best of which is the method of 

 forming the pendulum of bars of brass and steel 

 so placed, that the expansion of one corrects that 

 of the other, and thus preserves the centre of oscil- 

 lation always in the same place. This is called the 

 gridiron pendulum, from its resemblance to a grid- 

 iron. 



Deal-wood is found to expand very little in the 

 direction of the grain : hence it is much fitter for 

 pendulum-rods than metal. Baking, varnishing, 

 gilding, or soaking them in any melted matter, is 

 said to render them less accurate ; but rubbing on 

 the outside with wax and a cloth is recommended. 



CHRONOMETERS. 



Instruments for measuring time are called Chron- 

 ometers, and no class of machines has exercised 

 in a greater degree the genius of mechanics. 



The ancients were entirely unacquainted with 

 clocks and watches, and instead of them used sun- 

 dials, and also clepsydrce ; the latter were instru- 

 ments which measured time by the regular dropping 

 of water. Sand-glasses, constructed on the same 

 principle, are still used. It is not exactly known 

 at what period clocks were first invented; and 

 probably so complicated an instrument was the 

 result of repeated trials and improvements by 

 different persons. The oldest clock of which we 

 have any account, is that made by Henry de Wick, 

 a German, for the palace of Charles V. of 

 France. Its date is about 1364,. The wheels were 

 of iron, and very large ; and it required two men 

 to wind it up. It is also equally uncertain who it 



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